The field of the invention is synthetic resin polymerization and the present invention is particularly concerned with preparing polylauryllactam low in fisheye gel content by polymerizing lauryllactam at elevated temperatures in the presence of water and possibly of monoor dicarboxylic acids as chain regulators, the procedure taking place in a single step polymerization at temperatures ranging from 260.degree. to 340.degree. C. or in a two-step polymerization at temperatures ranging from about 260 to 340.degree. C. during the first step thereof and from about 200.degree. to 300.degree. C. in the second step of post-condensation.
The state of the prior art of polymerizing lauryllactam (nylon-12) may be ascertained by reference to the Kirk-Othmer "Encyclopedia of Chemical Technology", Vol. 16 (1968), pages 88-105, particularly p. 92 and U.S. Pat. No. 3,799,899, the disclosures of which are incorporated herein. The 2-mercaptobenzothiazole additives of the present invention are disclosed in U.S. Pat. Nos. 3,308,091; 3,459,702 and 3,558,553, and these disclosures are also incorporated herein.
It is known to the prior art to produce polylauryllactam by the polymerization of lauryllactam in the presence of water and optionally chain regulators wherein, in a first stage, the process is conducted under pressure at a temperature of between 200.degree. and 340.degree. C. and, in a second stage, between 270.degree. and 340.degree. C., as disclosed in German Published Application No. 1,495,149 and French Pat. No. 1,413,397. The products obtained according to this mode of operation possess a high fisheye gel content. The fisheye gels interfere with the production of threads, monofilaments, and especially films, causing opaque thickened portions and giving a non-homogeneous and unattractive appearance to the films, and furthermore, they make it difficult to imprint the material or make such imprinting even impossible.
It is likewise known to conduct the polymerization of lauryllactam in the presence of catalysts of the strong mineral acid type, such as phosphoric acid, phosphorous acid, or sulfonic acids at temperatures of between 280.degree. and 300.degree. C. in a single stage, as disclosed in German Unexamined Laid-Open Application Nos. 1,520,551; 1,907,032; and 1,495,147, or to carry out the process first at temperatures of above 300.degree. C. and then optionally conduct a post condensation at temperatures of below the melting point of polylauryllactam, as disclosed in U.S. Pat. No. 3,564,599. However, working with such strong, acidic catalysts has the disadvantage that the polylauryllactam thus prepared is subjected, during the processing thereof, to an increased hydrolytic degradation, since the acidic catalysts employed remain in the polymer and additionally, the fisheye gel content here again is very high.
Furthermore, it is conventional to subject lauryllactam to a preliminary polymerization in a first stage at a temperature of 265.degree. -320.degree. C. and then to a post polymerization at a temperature lower than in the first stage, namely 220.degree.- 265.degree. C., as disclosed in U.S. Pat. No. 3,799,899. According to the process of U.S. Pat. No. 3,799,899, a polylauryllactam having a low fisheye gel content is obtained. However, in general, longer reaction times are required than when working at higher temperatures, so that the space time yield is reduced. However, during disturbances in the operation, it can happen that the polylauryllactam is exposed to high temperatures for longer periods of time, so that then fisheye gels can occur.
In order to stabilize polyamides, numerous compounds have been known utilized essentially as heat or oxidation stabilizers as well as light stabilizers. A reduction in the gel formation has not been observed in this connection. German Published Application No. 1,694,473 merely mentions that copper compounds, in combination with lithium iodide, used for the heat stabilization of polyamides, simultaneously exhibit a reduced tendency toward gel formation. However, the polyamides are strongly discolored and the indicated examples refer exclusively to polyamide 6--6. Additionally, copper compounds are undesirable in finished polyamide articles coming into contact with foodstuffs, e.g., films for grocery packaging. According to U.S. Pat. No. 3,715,340, alkyl ammonium salts or alkylene bis(ammonium) salts of an aromatic carboxylic acid are added prior to polymerization to prevent the gel formation in polyamide 6. However, it is known that carboxylic acids and amines formed during the polycondensation from the ammonium salts are used during the production of polyamides for chain regulation, as disclosed in "Kunststoff-Handbuch", Vol. IV, Vieweg/Mueller, "Polyamide", pp. 19 et seq., 1966. The carboxylic acid reacts with the amino groups of the polymer and terminates the chain growth, whereas the amine component of the salt can react with the carboxylic acid groups of the polymer. Thus, the ammonium salts are completely incorporated into the polymer, i.e., with an addition of, for example, 0.5 molar percent, it is no longer possible to exceed a certain maximum intermediate molecular weight.
Suggestions have furthermore been advanced for obtaining polylauryllactam of a low fisheye gel content wherein 0.05- 1.5 percent by weight of 2-hydroxyphenylbenzoxazole is contained in the polylauryllactam, as disclosed in U.S. Pat. No. 3,951,909.